I am glad to see that you are incorporating weight training into your fitness program. Like you, my father is in his 60's and has also begun doing a bit of lifting. Always a naturally strong guy, his body had begun to deteriorate quite rapidly over the last year or so - especially his legs. While previous cardiovascular conditioning had certainly improved his health status and helped him lose about twenty pounds, the strength training has allowed him to continue to play golf at a fairly high level - he can still drive the ball over 300 yards! In your case, strength training will allow you to continue riding with your family for many years to come. In fact, if I were forced to prescribe either cardiovascular or strength training for an otherwise healthy, older individual I would choose strength training. Here's why: .....
...... Between twenty and fifty, adults lose about 5-10% of their muscle mass. Beyond fifty, however, muscle loss (atrophy) increases exponentially (by an additional 30-40%!) with strength also decreasing at a rate of nearly 15% per decade - most notably in the lower body. In turn, this loss of lean mass and strength leads to a decrease in resting energy expenditure (i.e. lower "metabolic rate") and whole body fat oxidation (fat "burning") as well as a reduction in physical activity.
These metabolic changes are likely to result in weight (fat) gain, increasing the risk of developing high cholesterol, insulin resistance, and cardiovascular disease. Further, bone mineral density is related to muscle mass and strength in older adults, implicating muscle atrophy in the development of osteoporosis. Therefore, the primary focus of resistance exercise beyond fifty should be the induction of muscle growth (hypertrophy) in an effort to counteract muscle atrophy.
Of secondary concern but important nonetheless, is the decline in muscle power output with age. The blunted capacity to contract muscle forcefully and quickly can increase the risk of falls. Given your health history and activity of choice, you might want to include some low-resistance, high-velocity training into your program to reduce your chances of injuring yourself in the future.
Finally, studies have also shown that increases in strength following resistance training are related to increased time to exhaustion in endurance activities with little or no increase in aerobic capacity (VO2). This is why resistance training is so important for ALL athletes - even those involved in "aerobic" sports. One possible explanation for this relationship is that less muscle activation would be needed to perform a task when a muscle is stronger, hence delaying fatigue.
As you can see (if you didn't already), resistance training is essential for the older athlete.
In looking at your program three things instantly jump out at me:
1. The exclusive use of machines
2. Light-weight, high-repetition training
3. One set per exercise
The first point is one of great debate among trainers. I am a firm believer in free-weight training for developing strength and power (i.e. squats, deadlifts, power cleans, etc.) although machines do offer a nice stable platform for developing muscular endurance. As fatigue sets in you are at less risk for hurting yourself. The drawback to machine training is that it works the muscles or groups of muscles in a less-integrated fashion with respect to the rest of the body and with little demand for balance and stability. Inter-muscular coordination, balance, and stability decline with age as well so I would encourage you to incorporate some free-weight training into your routine.
Secondly, while light-weight, high-repetition training is great for building muscular endurance, it does nothing to build strength or power. On the other hand, increased strength, as previously noted, does exert an influence on endurance. While it's not necessary to drop the high-repetition training altogether, I would substitute a "heavy" day for one of your regular training sessions.
Finally, one set provides too few exposures to the training stimulus to be of maximum benefit. Most studies have shown multiple-set training to be superior to single-set training in all but the extremely weak. This is one variable that you can change quite easily as your fitness level improves.
As was discussed in the preceding paragraphs, one of the aims of resistance training for older individuals is to stimulate muscular growth. The intensity for promoting hypertrophy should approach 60-80% (80% in more highly trained individuals) of your one-repetition maximum (1RM) with a volume ranging from 2-4 sets of 8-15 repetitions per exercise. The loads you are using at the moment fall short of those necessary to build muscle.
So, how do we put this all together?
First, substitute at least one of your normal training days with a "heavy" day. I would start with a weight that causes momentary muscular failure at about 10 repetitions. Later on you could add a second day leaving one day for high-repetition training.
Next, add a second set to each exercise and work your way up three or four over the coming weeks.
Finally, as your strength increases, add in some low-resistance, high-velocity movements to develop muscular power. These include low-level plyometrics such as medicine ball throws, jumping, etc. Do as much as your surgically-repaired knee will allow and, of course, always follow the advice of your doctor.
One last thing, continue to do your cardio but be sure to limit it to no more than three days per week to minimise impairment of strength and muscle mass gains.
Reprinted with permission from Steven Bubel.   For more great Q&A's, visit Steven's website at http://www.mxconditioning.com